A push type driving belt is known in the prior art to be used as a power transmission belt in a belt type continuously variable transmission. In general, the push type driving belt is formed by annularly juxtaposing a plurality of plate member called an “element” or a “block” in a same orientation, and by fastening the juxtaposing plate members by an annular member called a hoop or band to be called a “ring” hereinafter). Side faces of the element are inclined to serve as flank faces to be fit with a V-shaped groove of the pulley. Therefore, the flank faces thus formed are contacted with a surface of the V-shaped groove, and a torque is transmitted at the contact portion between the element and the pulley.
When the driving belt thus structured is clamped by the pulley, a force is applied to the element in the direction to push the element toward an outer circumferential side. Consequently, the ring fastening the element is strained. In this situation, when a driving pulley is driven by a torque, the ring thus clamped by the driving pulley is driven together with the driving pulley. Since the driving belt is applied to the driving pulley and the driven pulley, the elements in a straight region between the drive and driven pulleys are pushed by the elements sequentially pushed out of the drive pulley. The elements thus pushed in the straight region are then pushed into the V-groove of the driven pulley sequentially to be rotated together with the driven pulley. That is, the driven pulley is rotated by the element thus pushed into the V-groove by the following elements. Consequently, the torque is transmitted from the drive pulley to the driven pulley.
An example of the push type driving belt is disclosed in a publication of Japanese patent No. 3406283. Specifically, the belt for continuously variable transmission taught by Japanese patent No. 3406283 comprises a metal ring assembly formed of a plurality of endless metal rings laminated one on another, and a large number of metal elements each having a ring slot into which said metal ring assembly is fitted. The belt taught by Japanese patent No. 3406283 is applied to a drive pulley and a driven pulley to transmit the torque between those pulleys. According to the teachings of Japanese patent No. 3406283, the metal rings and a saddle face of the ring slot supporting the innermost metal ring each have a crowning, and a crowning radius of the metal rings is set smaller than a crowing radius of the saddle face.
Another example of the driving belt is disclosed in Japanese Patent Laid-Open No. 01-312247. Specifically, Japanese Patent Laid-Open No. 01-312247 discloses a transmission device with pushout type lateral link for continuously variable transmission (i.e., a driving belt). The driving belt taught by Japanese Patent Laid-Open No. 01-312247 comprises at least one endless flexible connecting element (i.e., a ring) which is quasi-inextensible longitudinally, and a non-metallic transverse link having a groove to hold the ring. According to the teachings of Japanese Patent Laid-Open No. 01-312247, a friction between the ring and an inner walls erected on both sides of the saddle face is reduced by coating the ring by a resin with a low coefficient of friction. In addition, according to the teachings of Japanese Patent Laid-Open No. 01-312247, the non-metallic ring is formed of elastomer or rubber.
Meanwhile, Japanese Patent Laid-Open No. 2001-317594 discloses a thin metallic ring of metallic belt. According to the teachings of Japanese Patent Laid-Open No. 2001-317594, the thin metallic ring assembly is formed by overlapping a plurality of thin metallic in its thickness direction, and the thin metallic ring assembly thus formed is used to form a metallic belt (i.e., a driving belt) together with a plurality of blocks (i.e., elements). According to the metallic belt taught by Japanese Patent Laid-Open No. 2001-317594, in order to reduce a stress generated at a contact portion between a pulley and the ring, a curved surface is formed on a portion of the inmost ring to be contacted with the pulley.
In addition, Japanese Patent Laid-Open No. 2002-168305 discloses an element for continuously variable transmission belt. Specifically, the element taught by Japanese Patent Laid-Open No. 2002-168305 is a plate-like element to be used to form a continuously variable transmission belt together with a band ring. The element taught by Japanese Patent Laid-Open No. 2002-168305 comprises a body part having a saddle face to which the ring abuts, and a pillar part extending from the saddle face. According to the teachings of Japanese Patent Laid-Open No. 2002-168305, a curved face is formed on the saddle face to extend circularly in the cross direction of the belt. In order to distribute a stress at a contact portion between the ring and the element, the curved face comprises a main face of a curvature radius Ra to which the ring mainly abuts, and an end face of a curvature radius Rb formed on the pillar side of the main face.
The push type driving belt, for example, the belt taught by Japanese patent No. 3406283 is generally used in a transmission mechanism requiring a comparatively large torque transmission capacity such as a belt type continuously variable transmission. Therefore, the metal ring of the push type driving belt is required to have two contradicting characteristics, such as: a sufficient strength or hardness to fasten the element in a circular manner; and flexibility or bendability to be applied to the pulleys smoothly. In order to fulfill the above-explained requirements, the metal ring assembly of the driving belt of Japanese patent No. 3406283, a layered ring 100 shown in FIG. 7 formed by overlapping a plurality of single layered thin metal ring 101 and so on are used in the prior art. The ring thus formed has not only a high strength but also flexibility.
As described, the driving belt according to Japanese patent No. 3406283 comprises the crowning on the saddle face of the element. Therefore, the ring is positioned at the center of the saddle face by frictional force acting between the saddle face and the innermost surface of the ring when the driving belt is driven. Accordingly, a side end face of the ring can be prevented from being contacted with a neck portion or a column of the element, or a V-groove of the pulley. For this reason, according to the teachings of Japanese patent No. 3406283, uneven wear of the ring and deterioration in durability of the ring can be prevented.
However, in case of using the metal ring assembly of Japanese patent No. 3406283 or the layered ring 100 in the driving belt, a slippage between the singly layered rings will occur inevitably when the driving belt is driven to transmit the power. Therefore, power transmission efficiency of the driving belt has to be degraded by such internal slippage of the layered ring.
On the other hand, according to the driving belt taught by Japanese Patent Laid-Open No. 01-312247, the ring is formed of elastic material such as rubber, resin etc. Therefore, according to the teachings of Japanese Patent Laid-Open No. 01-312247, a single layered flexible and monolithic ring can be formed, and the ring thus formed can be applied to the pulley smoothly. The above-explained internal slippage of the metallic layered ring can be prevented by using the single layered monolithic ring. Therefore, according to the driving belt taught by Japanese Patent Laid-Open No. 01-312247, a transmission loss resulting from the internal slippage of the ring can be avoided.
However, although the ring thus formed of the elastic material such as resin and rubber has an appropriate flexibility, the hardness thereof is relatively lower than that of the metallic ring. Therefore, in this case, a load derived from a tension of the ring or the like may be applied to the saddle face of the element unevenly. For example, in case of fastening the elements of Japanese patent No. 3406283 having the crowning on the saddle face using the ring made of rubber or resin whose hardness is relatively low, a frictional force acting between the saddle face of the element and the innermost surface of the ring has to be unstable. Therefore, the ring may not be positioned in the center of the element accurately.
Thus, in order to prevent uneven wear of the ring and deterioration in durability of the ring by avoiding a transmission loss resulting from an internal slippage of the ring while positioning the ring properly with respect to the element, it is necessary to improve the conventional push type driving belt, which is formed by fastening the metallic elements in a circular manner, and which is capable of transmitting comparatively large torque.